ABSTRACT People with the genetic disorder phenylketonuria (PKU) struggle with the lifelong challenge of maintaining restricted blood phenylalanine levels in order to avoid severe neurological sequelae. The large inter-person variation in phenylalanine metabolism can make achieving an appropriate therapy regimen for a given individual a lengthy and difficult process, and can be especially challenging for young children, adolescents, and women during pregnancy. Measuring patient phenylalanine levels is a critical component of assessing the efficacy of a therapeutic regimen. However, current phenylalanine testing is laboratory-based, requiring several days from sample collection to patient receipt of their blood phenylalanine level. This time lag is the critical barrier to effective therapy monitoring for PKU patients. There is an unmet need for a tool that empowers PKU patients with the ability to guide and personalize their therapy during each stage of their life. The consequences of not meeting this need are disability, lost time at work, and diminished quality of life for this often neglected population. The objective of the proposed project is to develop and validate a paper-based phenylalanine test for use by PKU patients wherever they are located. The proposed test would enable PKU patients to sample their blood at the recommended 2?3 hours after a meal and provide blood phenylalanine levels within 10 min of sampling. The proposed project would have high significance for the health of PKU patients and would serve as a model of effective patient-driven therapy monitoring that could be applied to other diseases that affect childhood development.